HIV-1 has a remarkable capacity to adapt to individual human hosts by escaping cytotoxic T lymphocyte (CTL) responses mediated by human leukocyte antigen (HLA) recognition. Though HLA types are highly polymorphic, and there is site-specific functional constraint to change in the virus, HIV-1 appears to escape HLA-restricted CTL (and possibly CD4 T helper cell) responses by genetic mutation. Here, the study of HLA-driven adaptation at a population level will be used to understand determinants of HIV-1 disease severity in the HIV-1 infected individual and to guide design of a vaccine that would most effectively overcome the adaptability of HIV-1 in human populations. The specific aims are to: 1) Characterize HIV-1 adaptation to HLA at a (host) population level to identify immune escape/adaptations across full length HIV-1 sequences in a large drug-naive cohort that has HLA and viral diversity representative of populations in the US; 2) Correlate HLA-driven adaptation to viral load; 3) Determine immunological and virological determinants of HLA-driven adaptation and viral load. The sites of HLA-associated selection will mark out in vivo epitope targets of immune responses and the viral load effects of escape at these sites serve as a quantitative measure of the balance between host immune pressure and genetic barrier to mutation unique to every specific escape/adaptation. The information will be used to (i) correlate in-vivo HLA class I selection effects with assayed ex-vivo CTL responses (ii) define novel CTL and CD4 T helper epitopes (ii) measure magnitude, phenotype and T cell receptor (TCR) characteristics of T cells that favour or mitigate specific adaptations and (iii) measure the replicative fitness cost that constrains immune selection at these sites; 4) Design a 'population-optimised' HIV-1 vaccine. The results will be used to determine how well the immune responses induced by a given vaccine would recognise diverse, variably HLA-adapted HIV-1 strains at those epitopes most critical for the prevalent HLA types of the study cohort.